1. Field of the Invention
This invention relates generally to photonic integrated circuits (PICs) and more particularly to the deployment of various kinds of electro-optic amplitude varying elements (AVEs) and/or electro-optic multi-functional elements (MFEs) integrated in monolithic photonic integrated circuits (PICs).
2. Description of the Related Art
This disclosure relates to photonic integrated circuits or PICs and the active and passive elements that may be integrated in such circuits, in particular, elements that are in addition to the primary functional elements comprising the circuits. For example, in the above incorporated patent applications, there are disclosed transmitter photonic integrated circuits or TxPICs and receiver photonic integrated circuits or RxPICs employed in optical communication systems or networks. The TxPICs minimally include, in monolithic form, a plurality of signal channels that each includes a modulated source having a unique emission wavelength or frequency, with their outputs coupled to an optical combiner that combines modulated source signal outputs into a single WDM signal for output from the chip. The RxPICs minimally include, in monolithic form, an input to an optical decombiner with multiple outputs each coupled to a photodetector. This disclosure fundamentally relates to the addition of active elements to these circuits and these additional elements are collectively referred to as electro-optic amplitude varying elements (AVEs) and/or electro-optic multi-functional elements (MFEs) to perform various other functions in the operation of the circuits.
An optical transmission network or an optical transport system is limited in performance due to several issues. The primary issues are optical signal-to-noise ratio (OSNR) at both the optical transmitter and receiver, the Q at both the optical transmitter and receiver, and the dynamic range of the optical receiver, i.e., the level of ability to receive distorted channel optical signals and still interpret the data represented by the information modulated on the channel signals sent from the optical transmitter. This level of dynamic range at the optical receiver is a composite of many factors, such as, for example, the gain flatness of an optical amplifier just prior to the input of the optical receiver, which amplifier is usually a EDFA, the sensitivity variation in the optical transmitter and receiver, launch power variations in the optical transmitter, wavelength dependent losses and insertion losses in the optical transport system. The accumulative effect of the foregoing is to limit the overall reach of the optical transmission system or, alternatively, to increase the cost of the system. The optical receiver dynamic range is ultimately dictated by the noise and saturation effects of the signal channel photodetectors, which receive a demultiplexed optical channel signal for conversion into an electrical signal, and the noise and saturation effect of the transimpedance amplifier (TIA) coupled to receive the photocurrent channel signal. This noise and saturation effect can be quite large such as 5 dB to 15 dB, for example.
An important part of current day wavelength division multiplexing (WDM) transmission systems is the monitoring of system parameters that are indicative of impairments in the system such a per channel signal power, per channel wavelength stabilization, channel power level across an array of signal channels with an eye toward power equalization as well as gain tilt across the channel gain spectrum with gain tilt being significantly imposed on the channel signals by optical fiber amplifiers, such as EDFAs.
Also, in a WDM communication system, since each modulated signal channel is allocated a different wavelength that together approximate a standardized wavelength grid, the different wavelengths experience different delay effects in propagation in the optical medium or fiber as well as nonlinear effects of stimulated Raman scattering in the fiber so that when the channel signals are received on the optical receiver side of the system, the modulated channel signals have experienced chromatic dispersion due to both the characteristics of the fiber medium and also the gain characteristics and gain slope of a mid-span optical fiber amplifier. Thus, it is desired that optical power levels of the channel signals be equalized as they emerge from the transmitter. Even if the transmitted channel signals are equalized, they arrive at the receiver distorted with variations among the optical signal levels resulting in an unacceptable level of transmission errors. The transmission characteristic brought about by the foregoing effects is measured by the optical signal-to-noise ratio or OSNR as viewed at the optical receiver. The OSNR is improved by the deployment of pre-emphasis technology by adjusting, on the transmission side, the amplitude profile of the channel signals across the channel wavelength spectrum where such adjustment takes into account the dispersion characteristics of the fiber medium and/or the gain characteristics of link optical fiber amplifiers. The gain characteristics of an EDFA are typically strongest in the center of its gain spectrum so that in the pre-emphasized state, the pre-emphasis performed on the transmitter side would be an opposite gain spectrum across the channel signal array where the center channel would have the lowest power and extending to either side of the center the gain profile across those channels would monotonically increase so that the outside channels of the array will end up with the most initially applied gain.
In order to either equalize the channel or transmission signals, attenuators or amplifiers in combination with attenuators are deployed. It is known in the art to utilized variable optical attenuators (VOAs) by themselves or in combination with semiconductor optical amplifiers (SOAs) particularly for the purposes of providing signal equalization across an array of signals. A good example of the state of the art is disclosed in U.S. Pat. No. 6,271,945 where discrete devices are employed for discrete trains of electro-optic elements or components for each signal channel as seen in FIGS. 9 and 10, for example, of this patent. The elements comprise a discrete array of laser sources operating at different channel wavelengths and each coupled to an external modulator which is coupled, via a coupler to a corresponding attenuator in one embodiment (FIG. 9) or to a correspond amplifier (FIG. 12) in another embodiment. After multiplexing of the signal channels, a portion of the signal is tapped off to a spectrum analyzer to determine the power level of each channel signal. If any adjustment is necessary to equalize the channel signals relative to one another, a control circuit is employed to adjust the attenuation or gain level of a respective signal channel via its attenuator or amplifier to bring the channels back into equalization. In U.S. Pat. No. 6,282,361, an integrated multi-channel optical attenuator comprising an array of attenuators, e.g., a Mach-Zehnder interferometer (MZI), is disclosed where the channel signals provided as an input to the attenuator are equalized across a channel array via a per channel attenuator.
While the interest in this application is the deployment of such optical gain equalizing elements or components in monolithic photonic integrated circuits or PICs, this is not to say that there have not been suggestions of such in the art. For example, in FIG. 13 of U.S. Patent Application Pub. No. US2002/0109908A1, published Aug. 15, 2002, a monolithic device that includes a double pass multiplexer/demultiplexer that has a common input/output is illustrated in combination with a SOA and a VOA in each signal channel which, respectively, increase and decrease signal intensity so that the overall intensity level of all signals across the channel signal array are substantially uniform. The SOA in each channel increases the gain in the channel by increasing the bias on the amplifier which induces population inversion to bring about optical gain to a channel signal traversing the amplifier. In a VOA, the application of an applied negative or reverse junction bias brings about optical absorption and the amount of absorption of a channel signal traversing the attenuator is determined by the amount of reverse bias that is applied to the device in conjunction with, of course, the absorption length of the device. As indicated in this publication relative to one mode of operation, the frequency response of the VOAs is higher compared to that of SOAs so that the channel signals can be first amplified to higher values greater than the required minimum so that the rapid response of the several VOAs can be utilized to quickly achieve equalization across the array of channel signals. The publication, WO02/098026A1, published Dec. 5, 2002, shows a similar double pass device multiplexer/demultiplexer but without the deployment of SOAs.
Another aspect in the utilization of PICs is the optimum placement of integrated amplitude varying elements (AVEs) in the signal channels of an array of modulated sources, such as an integrated modulated laser in each channel on the PIC or an integrated laser source and external electro-optic modulator in each channel on the PIC. AVEs such as SOAs, VOAs, ZOAs (combination SOA/VOAs) or monitoring photodetectors (PDs) functioning also as a reverse bias AVEs or like VOAs when placed in different locations in PIC signal channel paths can have detrimental affects on the channel modulated signal. As an example, in the case where the array of laser sources, whether DFB lasers or DBR lasers, form a plurality of signal channels in a transmitter photonic integrated circuit (TxPIC), it may be desired to operate the laser sources at a constant bias current above their respective thresholds while providing a feedback system to stabilize their wavelength operations over life such as disclosed in Pub. No. US 2003/0095736 A1, supra. In order to accomplish constant output from the constant bias current laser sources over life, it is necessary to control their power output across the channel array to be substantially uniform. In order to accomplish this task, some type of AVE can be included in each signal channel path so that the output power of modulated signals from each channel to the on-chip optical combiner are all substantially at the same power level. However, the added channel AVEs may have some affect one the optical modulated signal shape and the signal optical spectrum so that it becomes important as to where such AVEs may be placed in the signal channel paths to achieve optimum performance in terms of modulated signal output substantially unaffected by AVE operation.